Start by preheating your oven to 375 degrees F (191 degrees C). If using frozen spinach, make sure it is fully thawed and squeeze it well to drain any excess liquid. If you are using broccoli florets, bring a pot of water to a boil, add the broccoli, and let it simmer for 1–2 minutes until it turns bright green. Then, drain the broccoli well. Properly draining the spinach or broccoli ensures your casserole won't be watery.

In a large bowl, combine the cooked chicken breast (diced or shredded), chopped cooked bacon, drained spinach or broccoli (from Step 1), minced garlic, ranch dressing, 1/2 cup shredded mozzarella cheese, and 1/2 cup shredded cheddar cheese. Stir thoroughly until all the ingredients are well incorporated. Then, transfer the mixture to a 9x13 inch (23x33 cm) glass or stoneware casserole dish. You can also mix all the ingredients directly in the casserole dish if you prefer less cleanup. I like to add a bit of ranch seasoning at this stage for even more ranch flavor.

Evenly sprinkle the remaining 1/2 cup mozzarella cheese and 1/2 cup cheddar cheese over the top of the casserole. This will create a golden, bubbly crust once baked.

Place the casserole in the preheated oven and bake for about 15 minutes, or until the cheese is melted and the dish is hot and bubbling. Once out of the oven, let it rest for a few minutes before serving to allow everything to set. I like to serve this dish hot, when the cheese is at its meltiest and most delicious.